Here's my story, if anyone's interested...
I committed to a late summer's worth of fill-in gigs for a local band after their bass player suddenly quit... there were 8 or 9 total.
I rearranged a bunch of things, memorized about 50+ songs from their set list because they didn't allow charts, took time out of my life to go to 5 different rehearsals, and turned down a few other offers because I had already penciled in these other dates.
After two of the gigs, they called me and told me they got lucky found someone and wouldn't need me to do the rest of the gigs, as if it wasn't a big deal... well, I'm sorry but that doesn't fly with me.
I was pissed and told them that the new guy can wait... and if he can't, then they need to pay me for my time and for the trouble of rescheduling/cancelling other gigs.
We eventually settled on an agreement where I would do 4 of the gigs and they paid me out about half of what I would have made on the remaining 4 or 5 gigs.
Two of the people in the band thought I was the biggest a$$hole in the world for even suggesting this and two of the other band members stuck up for me... however you look at it, I did them a favor and spent a lot of time making sure the transition would be seamless.
I'm honestly a nice guy for the most part, but I don't do charity work for other bands unless the gig is specifically for charity.